1. Field of the Invention
The present invention relates to a light-receiving module used in a light-receiving apparatus for light transmission. In particular, the present invention relates to a light-receiving module used in a subcarrier multiplexing transmission system in which distortion and noise should be low.
2. Description of the Related Art
In Subcarrier multiplexing (hereinafter, referred to as SCM) transmission, a carrier wave is modulated by the signal that is to be transmitted and then the intensity of a semiconductor laser beam is directly modulated by the modulated carrier, thereby transmitting light. In such a SCM transmission, multipath represented by multiple reflection is one of the causes of deterioration of transmission characteristics such as noise and distortion.
For example, when an optical signal for SCM transmission is transmitted through a transmission path having a plurality of reflecting points, an optical signal received by a light-receiving apparatus provided at the end of the transmission path includes a direct light which reaches the light-receiving apparatus without being reflected, and a delay light which reaches the light-receiving apparatus by being reflected even number of times at the plurality of reflecting points. This means that the transmission path from a light-transmitting apparatus to a light-receiving device of the light-receiving apparatus has paths traced by the direct light and traced by the reflected light. The plurality of paths are referred to as "multipath". In particular, such a transmission state can often be seen in radio transmission systems.
The delay light travels in a long optical path as compared with the direct light since the delay light is reflected at each reflecting point. Therefore, a time delay occurs in the arrival time of the delay light. Normally, the reflection amount at each reflecting point is not very big. For example, the reflection amount at an open end of the transmission path is about -15 dB, and that in a normal optical connector is in the range of -25 to -30 dB. Therefore, the difference in the level of optical power is caused between the direct light and the reflected light to some degree.
The direct light and the delay light generated by the presence of the multipath are the same signal, except for the arrival time and optical power level. Therefore, the lights interfere (delayed self-homodyne) with each other on a light-receiving face of the light-receiving device and generate beat, whereby beat also appears in an electric signal converted from the two optical signals at the light-receiving device. Particularly, the SCM transmission has such a problem that the beat interferes with the transmission as the noise and the distortion in the frequency region of the electric transmission signal, causing deterioration of the transmission characteristics. As for the deterioration of transmission characteristics due to the multipath reflection, experiments and theoretical analyses have been reported (ex. A. Lidgard and N. A. Olsson, "Generation and Cancellation of Second-Order Harmonic Distortion in Analog Optical Systems by Interferometic FM-AM Conversion", IEEE Photonics Technology Letters, Vol. 2, No. 1, pp. 519-521, 1990; J. H. Angenent, I. P. D. Ubbens, and P. J. de Waard, "DISTORTION OF A MULTICARRIER SIGNAL DUE TO OPTICAL REFLECTIONS" ECOC'91, IOOC' 91, WeC8-4, 1991).